The present invention relates to a technique for controlling saturation of integrated bipolar transistors.
The saturation of an integrated bipolar transistor causes an injection of current into the substrate (which is commonly connected to ground). This effect of saturation, beside representing an undue power consumption, may also have other consequences, all highly undesirable. Of course the problem becomes severe in the case of power transistors, which are commonly driven through a base terminal by a driving circuit. The driving circuit may comprise an operational amplifier, to the inputs of which a control signal is fed.
A representative application of this type of circuit employing a power transistor is that of a so-called series voltage regulator. FIG. 1 is a functional schematic diagram of this common circuit wherein, in the particular instance, the power transistor is a PNP transistor. The operational amplifier OP drives the base of the power transistor T1 in function of a control signal fed to its inputs. In this specific instance, this signal represents a voltage difference between the regulated output voltage V.sub.L and a reference voltage V.sub.R. When the V.sub.L voltage is lower than V.sub.R, the OP amplifier increases the output current I.sub.B that, fed to the base of T1, forces the transistor to increase its collector current I.sub.L, thus determining a desired increase of the output voltage V.sub.L. Conversely, V.sub.L is greater than V.sub.R, a reduction of the base current I.sub.B and therefore of the collector current I.sub.L and of the output voltage V.sub.L is obtained.
Of course there are physical limitations to the variation ranges within which such parameters may vary. In particular, by progressively reducing the input voltage I.sub.IN, a condition is reached eventually whereby the difference of potential (V.sub.CE) between the emitter and the collector of the power transistor T1, by dropping to a few hundred mV, brings the transistor to operate in a saturation region of its characteristic. Under these conditions of operation, the voltage regulator of FIG. 1, i.e. the bipolar transistor T1, ceases to provide a correct performance. In a regulator circuit as the one shown in FIG. 1, it becomes impossible to maintain an output voltage V.sub.L equal to the desired voltage V.sub.R, because to an increase of the base current I.sub.B of the power transistor T1 no longer corresponds an increase of collector current. The operational amplifier OP, in an attempt of maintaining V.sub.L equal to V.sub.R increases its output current up to the maximum level of its intrinsic regulating capabilities. This last effect has in turn other negative influences when the power transistor T1 is an integrated transistor. In this case, the transistor has a "fourth terminal", commonly referred to as the substrate terminal (schematically indicated with "S" in FIG. 1) which, for obvious reasons, should be kept at the lowest potential present in the integrated circuit (typically to ground potential). The negative effects caused by the saturation of an integrated power transistor T1 are essentially connected with the generation of a current I.sub.S through the substrate terminal, which is related to the base current I.sub.B by a gain factor. Therefore this current I.sub.S, which is injected in the substrate of the integrated circuit, is generally rather high, by considering that already the base current I.sub.B tends to be high under saturation conditions.
A saturating transistor may even invert the sign of its own collector's current if any component connected thereto can sustain it. As a consequence the current I.sub.L may flow into the collector of the power transistor T1. The presence of an inductive load (L) is a typical situation wherein an inversion of the collector's current may occur. In other circumstances, large capacitances are often connected in parallel to the load of a voltage regulator for obvious reasons. One reason may be the need of sustaining a certain current through the load also during brief interruptions of power supply to the regulator circuit. Therefore, under the above noted conditions of saturation, these "storage" capacitors may deliver a current (-I.sub.L) to the collector of a saturating power transistor, thus unduly losing stored electrical charge and reducing the time of "self-sustainment" of the current through the load in case of interruption of the power supply.
The substrate current I.sub.S should be added to the base current I.sub.B of the power transistor T1 for determining the sum of the emitter and collector currents, according to the relation: I.sub.S +I.sub.B =I.sub.IN -I.sub.L. As a consequence, a large value of the sum: I.sub.S +I.sub.B, determines a large "input" current I.sub.IN, as well as a high discharge current (-I.sub.L) of a load capacitance. Both these currents are highly undesirable because they may overload the current path, cause undue power dissipation and eventually reduce the period of interruption of the power supply that the powered circuits may tolerate before losing their functional ability.
Several devices are known for controlling saturation of an integrated bipolar transistor with the aim of relieving or preventing the above-noted consequences. An antisaturation circuit that is commonly used according to a known technique is schematically shown in FIG. 2. The transistor T2, which is in an OFF condition during normal operation of the circuit, switches ON when the voltage between the collector and the base of the power transistor T1 becomes equal to the threshold (base/emitter) voltage of T2. The current flowing through T2 is then used for reducing or switching OFF the output current of the operational amplifier OP which drives the base of T2. In this manner the problems caused by an uncontrolled increase of such a driving current when the power transistor T1 enters saturation are contained.
Basically, the principle on which the operation of these known antisaturation circuits rests is that a transistor may be defined to be working in saturation conditions when its collector/base junction is directly biased. The base/emitter junction of the antisaturation transistor T2 is connected in parallel to the collector/base junction of the power transistor and therefore the transistor T2 begins to conduct when such a collector/base junction of the saturating power transistors becomes directly biased.
This well known solution, though producing a satisfactory control of saturation, is not devoid of drawbacks. In view of the fact that the power transistor T1 generally carries relatively high currents, there may be voltage drops along the connection between its collector terminal and the output terminal (OUT). In order to prevent such voltage drops from falsifying the voltage "read" by the emitter of T2, this transistor must be physically located very close to the collector terminal of T1 (to minimize the length of the connection). This is not always easy to realize in the case of an integrated power transistor because such a device has already a relatively complex system of connections. The structure of the base terminal of T1 may present similar problems of layout (though somewhat simpler because the current I.sub.B is at least an order of magnitude lower than I.sub.L).
In general, according to the prior art, control of saturation is implemented by acting upon the cause of such a current, i.e. the occurrence of a directly biasing voltage (V.sub.BC) across the collector/base junction. However, it is also necessary to ensure that the current I.sub.S does not rise above a certain maximum value. For this reason, under critical working conditions and/or in presence of critical intrinsic physical parameters of a particular fabrication process, it is necessary to adjust the intervention threshold of T2 with a certain safety margin and in many occasions the power transistor T1 may be in a condition which is not yet of true saturation when the antisaturation circuit intervenes. A not yet saturated T1 imposes a lower limit (V.sub.CEmin) to the difference between emitter and collector potential which is higher than what would be ideally necessary for eliminating the negative effects of saturation of the power transistor, as noted above. A higher V.sub.CEmin voltage (as required by safety margin considerations) may have a remarkable importance, as in the case of a voltage regulator, because in turn it imposes a lower limit to the input voltage V.sub.IN during operation. In fact, it may be observed from FIG. 2, that V.sub.INmin =V.sub.CEmin +V.sub.R. In order to ensure the lowest V.sub.INmin as possible, it would be desirable that the equality: V.sub.CEmin =V.sub.CEsat be always verified, wherein V.sub.CEsat is the V.sub.CE of the power transistor T1 under mild (initial) saturation conditions, which would not yet cause the above-noted consequences.
On the other hand, fast transients may occur while the V.sub.IN voltage is within a normal working range for which the antisaturation transistor T2 should not intervene. Such transients propagate to the base of the power transistor T1 because its V.sub.BE is approximately constant. From this point, through the parasitic capacitance that exists between the collector and the base of the antisaturation transistor T2, the transients reach the collector of T2, which is connected to a high sensitivity input node of the driving operational amplifier OP. As said above, such an input signal modulates the output current, i.e. I.sub.B, which in turn acts upon I.sub.L and eventually on V.sub.L. In conclusion, the presence of the antisaturation transistor T2, with its parasitic capacitances, worsens the overall performance of the circuit in presence of fast transients on the supply lines. In many applications this aspect becomes very important because of the consequences that may ensue therefrom.
According to the disclosed innovations, the current injected into the substrate by a saturating transistor is used as information for exerting a limiting control of saturation. The present invention advantageously provides a system for controlling the saturation of an integrated bipolar power transistor, which may be easily implemented independently of the layout of the power transistor and wherein the control on saturation is readily adjustable in order to intervene when a certain tolerable level of saturation is surpassed.
It is a further object of the present invention to provide an improved voltage regulator circuit, having a low voltage drop and utilizing a bipolar power transistor as a series regulating element, driven by an operational amplifier, and provided with an improved circuit arrangement for controlling saturation of the power transistor used as series regulator.
Basically, the antisaturation system of the present invention, differently from the known systems, does not monitor the occurrence of a direct biasing of the collector/base junction of the power transistor as an indication of a saturation condition of the transistor. Instead, the system of the invention monitors a substrate current which is actually caused by the operation of the transistor in a saturation region of its characteristic. This is implemented by connecting a sensing resistance, generally of a small value, in the connection path between the substrate terminal of the power transistor and a ground node of the integrated circuit. The current which is eventually injected into the substrate by the power transistor when operating in a saturation condition, determines a certain voltage drop across the sensing resistance and this voltage is used as a signal confirming a certain saturation state. This voltage is compared with a reference voltage, for example by the use of an operational amplifier capable of generating an output current that is fed to an input of the driving circuit of the power transistor in order to exert a desired control on the saturation.
According to the system of the invention, it is no longer necessary to "disturb" the geometry of an integrated power transistor because it is no longer necessary to detect, as done in the known circuits, an "internal" operating voltage of the integrated transistor. An information in the form of a substrate current is not altered by the length (resistance) of the connection path between the substrate of the transistor and a sensing resistance, which advantageously may be physically realized far away from the integrated structure of the power transistor.
A corrective (limiting) action on saturation is effected by modifying the driving base current of the saturating power transistor, similarly to what is done in an antisaturation system of the prior art. By contrast, instead of monitoring the parameters or the parameter which causes saturation of the power transistor (i.e. its V.sub.BC), a physical effect of saturation, that is the current which is injected into the substrate, is monitored instead.
In this way it is possible to limit very precisely the degree of saturation, by adjusting the triggering threshold of a comparator. The power transistor is permitted to reach a certain predetermined tolerable level of saturation, irrespective of the working conditions and/or of the spread of electrical parameters typical of the particular large-scale fabrication process. In this manner, a very useful condition: V.sub.CEmin =V.sub.CEsat, wherein V.sub.CEsat can be extremely small, may be ensured, because the operating conditions of the power transistor are corrected only when strictly necessary to avoid undesirable consequences.
In the case of a typical application such as that of a series voltage regulator, transients which may occur in the input line V.sub.IN are not propagated to the inputs of the comparator which controls saturation, nor to the output node, and therefore the antisaturation circuit of the invention does not modify in any way the behavior of the voltage regulator in presence of transients.